The present invention relates to Automatic Repeat Request (ARQ) techniques for transferring data in fixed/wireless data networks.
ARQ techniques are commonly used in data networks to ensure reliable data transfer and to protect data sequence integrity. Data packets are encoded with an error detecting code, so that when a transmitter in the data network sends or transfers data packets to a receiver in the data network, the receiver receiving the data packets can detect corrupted, erroneous or lost packets and thereby request that the transmitter retransmit the affected data packets. The integrity of a data sequence is normally protected by sequentially numbering packets and applying certain transmission rules.
There are three main ARQ schemes: Stop-and-Wait; Go-Back-N; and Selective Reject (sometimes referred to as Selective Repeat). All three methods provide mechanisms for transferring packets to a receiver in a data network in an appropriate order. In terms of throughput efficiency as a function of the signal to noise ratio, generally Selective Reject is most efficient, Stop-and-Wait is least efficient, and Go-Back-N is intermediate. Also, various mixtures of the Selective Reject and Go-Back-N techniques exist, and fall between pure Selective Reject and pure Go-Back-N techniques in both efficiency and complexity.
With respect to Go-Back-N, several different variants exist which differ in terms of how they use positive acknowledgments (PACKs), negative acknowledgments (NACKs), retransmission timers, polling schemes, etc.
One type of Go-Back-N technique uses both PACKs and NACKs that have the following characteristics:
A PACK for a data packet having a sequence number N(R) gives a cumulative positive acknowledgment for data packets having sequence numbers before N(R), but does not positively acknowledge the data packet having the sequence number N(R), as shown for example in FIG. 1A.
The NACK positively acknowledges all data packets before the data packet it negatively acknowledges. The data packet which the NACK negatively acknowledges is indicated by N(R), as shown for example in FIG. 1B.
FIG. 2 shows a simplified ARQ transmitter window, in which five variables are used to keep track of a transmitter state. The five variables include: a bottom sequence number, BSN; a top sequence number, TSN; a maximum top sequence number, TSNMAX; an instant sequence number, ISN; and an expected sequence number, ESN.
BSN denotes the oldest packet in the transmitter buffer, and can also indicate that all packets before the BSN packet have been acknowledged or discarded. Packets prior to the packet indicated by TSN have been sent. ESN denotes the expected sequence number of a packet to be received. ISN indicates the sequence number of the next packet to be sent. When a packet is sent for the first time, TSN and ISN will be identical. However, when a retransmission is performed, ISN will start over from the first retransmitted packet and progress in consecutive order, one packet at a time, up to TSN. TSN cannot exceed TSNMAX, which is defined by the window size W. Assuming that a sequence number field has k bits, 2k different sequence numbers can be created. Thus, the maximum size W of the window shown in FIG. 2 is 2kxe2x88x921.
Operation of the Go-Back-N technique using both PACKs and NACKs can be envisioned by imagining a clockwise consecutive modulo 2k sequence numbering superimposed upon the circumference of the circles shown in FIGS. 3A-3D. FIG. 3A shows a circle indicating a state where no packets have yet been sent, and TSN, ESN, BSN and ISN all have the same value, i.e., point to the same packet. The circle shown in FIG. 3B indicates that (TSN-BSN) packets have been sent and also received, since ESN=TSN. An erroneous or lost packet causes ESN to stop progressing forward, although more packets have been sent. For example, in FIG. 3C packets up to the packet indicated by TSN and ISN have been sent, but ESN indicates a prior packet which was not received. After a packet is lost or an erroneous packet is received, the ARQ receiver sends a NACK to the ARQ transmitter to inform the ARQ transmitter about the lost or erroneous packet. The NACK includes a returned sequence number N(R) that is set equal to ESN, thereby acknowledging that all previous packets were correctly received. BSN and ISN are set equal to ESN (and N(R)) so that BSN moves forward and ISN moves backward to the sequence number representing the lost or erroneous packet. Thereafter, as shown in FIG. 3D, ISN and ESN move forward together as the lost or erroneous packet is retransmitted, and as the succeeding packets are also retransmitted.
FIGS. 4A-4D illustrate use of a PACK. For example, FIG. 4A shows a state where nothing has yet been sent, and TSN=ISN=BSN=ESN. FIG. 4B shows a situation where all sent packets have been correctly received. FIG. 4C shows that a timer-initiated PACK is sent, conveying the sequence number N(R) of a packet between BSN and TSN=ESN=ISN. As shown in FIG. 4D, after the PACK is sent, BSN is set to N(R).
Sending PACKs ensures that sequence number starvation does not occur. Since TSN may not pass BSN, if the transmitter does not receive PACKs, it may continue to send data packets up to TSNMAX. However, if data packets up to TSNMAX are sent but no PACKs are received, then TSNMAX cannot progress and sequence number starvation occurs. The transmitter must wait until it receives a PACK, which will allow BSN and thus TSNMAX to progress.
FIG. 5 shows a general example of an ARQ data packet 510. The packet 510 typically includes an ARQ header 512 and a data portion 516. The header 512 contains a k-bit sequence number 514, and can be located at the front of the packet 510 as shown in FIG. 5, or at any predefined position within the packet 510.
FIG. 6 shows an exemplary ACK message 610, with an identifier field 612 that identifies the responding terminal sending the ACK message 610, a NACK/PACK type indicator 614 indicating whether a PACK or a NACK is being sent, and finally a sequence number field N(R) 616 that indicates for which sequence number the ACK message 610 is valid.
In a Selective Reject scheme, a sender window having a size of 2kxe2x88x921 or less is normally used in order to avoid certain ambiguities which appear in conjunction with an automatic (timer-initiated) retransmission. The receiver window size in a Selective Reject scheme can include up to 2kxe2x88x921 positions, instead of just one position as in a Go-Back-N scheme. In Selective Reject a range of packets can be received since the receiver window can include up to 2kxe2x88x921 positions.
As long as packets are received correctly, they are sent or forwarded to the next higher layer. When an outstanding packet is detected, i.e., a packet that has been sent but not received or not correctly received, the sending of subsequent packets up to the higher layer is halted and a list of correct and missing packets is built up. A NACK is used to initiate a request for a retransmission of the outstanding packet or of a multitude of outstanding packets. When the first detected outstanding packet is correctly received, that packet and all subsequent packets are sent to the higher layer, until the next outstanding packet is detected and the process repeats with respect to the new outstanding packet.
FIG. 7A, for example, shows a situation wherein three packets are outstanding. The outstanding packets are denoted by ESN1, ESN2 and ESN3. The receiver sends one or several NACKs indicating the sequence number of these outstanding packets. In FIGS. 7B and 7C, the transmitter has received the one or several NACKs and in response retransmits the outstanding packets. The transmission of new packets can proceed to the TSNMAX limit, which of course can also occur when no NACKs are received. In particular, FIG. 7B shows a situation where ESN1 has been retransmitted and correctly received, and ESN2 is currently being retransmitted. BSN has also been set to ESN1. In other words, the NACK for ESN1 functions as a cumulative positive acknowledgment for packets preceding ESN1, and BSN is adjusted accordingly.
Sometimes, NACKs fail to reach the transmitter for unknown reasons. In such a situation, after a specified or predetermined time has expired, packets in the sender buffer that have not been acknowledged (by either a NACK or a PACK) can be automatically retransmitted.
NACKs can be efficiently sent by sending a NACK and explicitly indicating the oldest NACK""s sequence number, here represented by ESN1, and using a bitmap to thereafter represent correctly received packets and missing packets. This type of NACK performs a cumulative PACK for the packets preceding the sequence number which is NACKed. Other NACK options can also be used, for example NACK options where a cumulative positive ACK is not performed or sent for the packets preceding the sequence number which is NACKed.
The Selective Reject and Go-Back-N techniques differ in the sense that Selective Reject does not require packets to be sent in any particular order, while the Go-Back-N receiver needs to receive packets in consecutive sequence number order.
Normally, in data networks it is desirable to transfer all packets without any packet loss. Sometimes, however, sending significantly delayed packets provides no benefit, for example where the delay causes the information in the packets to become outdated and therefore useless to the receiver. Examples of delay sensitive applications are, e.g., telephony, video conferencing and delay sensitive control systems.
Furthermore, non-time-critical applications commonly issue higher level retransmissions whenever they detect an absence of responses or acknowledgments from the receiving end, which can give rise to situations where the ARQ buffers are filled with not-yet-successfully transmitted data, and/or with newly retransmitted data. This can be avoided if data is associated with a validity time, and the validity time is set to be slightly shorter than the retransmission time for the application. However, in practice it can be difficult or impossible to discern which retransmission time is used, since the lower layer (LLC) is unaware which application is at the top level. In such a situation one has to assume a certain application and specially design the communication system based on that assumption.
For certain service classes and after a certain transfer delay time, discarding of data packets is allowed in Asynchronous Transfer Mode (ATM). An ARQ in conjunction with ATM can use transfer delay information provided by the ATM layer in order to adjust connection-specific discard timers in the ARQ function. However, the ARQ in the receiver may detect missing or incomplete packets and require retransmission.
In summary, current ARQ methods do not recognize and allow for situations where data packets have a limited lifetime, and therefore fail to minimize bandwidth usage by not sending (or resending) significantly delayed or outdated data packets.
In accordance with exemplary embodiments of the invention, ARQ techniques are provided that minimize bandwidth usage by accounting for data packets that have an arbitrary but limited lifetime. The lifetime can either be assumed to be fixed, or can be deduced from ATM layer information. In particular, exemplary embodiments of the invention variously illustrate enhanced Go-Back-N and also Selective Reject techniques that discard outdated data packets, and which embody principles that can be applied to Stop-and-Wait techniques to discard outdated data packets.
In accordance with an embodiment of the invention, a bit is set in the ARQ header to force the receiver to accept packets subsequent to one or more erroneous or unreceived packets that have been discarded and not resent.
In accordance with another embodiment of the invention, when a NACK has been received and data packets have been discarded, sequence numbers are reassigned to the non-discarded data packets so that a transmitted stream of the non-discarded packets will have consecutive sequence numbers.
In accordance with another embodiment of the invention, at a packet discard the transmitter monitors the receiver state. If a packet is expected which has already been discarded, then the transmitter resynchronizes by renumbering data packets or by commanding the receiver to accept an arbitrarily chosen sequence number.